Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises a housing in which is provided an exhaust gas driven turbine wheel mounted on a rotatable shaft connected downstream of an engine outlet manifold. A compressor impeller wheel is mounted on the opposite end of the shaft such that rotation of the turbine wheel drives rotation of the impeller wheel. In this application of a compressor, the impeller wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems.
The compressor impeller is mounted in a compressor housing which comprises a cover plate, a portion of which closely follows the contours of the impeller blades and a portion of which defines an annular inlet passageway, and a diffuser flange that is fixedly connected between the cover plate and a bearing housing that retains the bearings for the compressor and the turbine.
There is an ever-increasing demand for turbochargers of higher performance, particularly with vehicles of high horse power. In order to meet this demand it has been necessary to manufacture the compressor impeller from titanium so that the compressor can withstand the high pressure ratios and arduous operating conditions. A disadvantage of an impeller made from titanium or another high density material (e.g. stainless steel) relative to the current aluminium alloy impellers is that the increased density makes the impeller more difficult to contain in the event of its failure. Failure of the compressor impeller can occur through defects in the titanium, consistent use of the turbocharger at speeds in excess of its top speed limit, or fatigue damage to the material caused by continually cycling between high and low turbocharger speeds in extreme duty cycles. When the compressor impeller fails in use it is desirable to contain the radially projected fragments within the compressor housing to reduce the potential for damage to the turbocharger or injury to personnel. Generally small fragments are relatively easily contained but larger fragments tend to damage the compressor housing or diffuser flange through their force of impact. At particular risk is the connection between the diffuser flange and the bearing housing. If the two are separated oil leakage from the bearing housing can occur thereby increasing the risk of fire in the engine compartment or failure of the engine.